For the beet chips: Preheat the oven to 300 degrees F, or preheat a convection oven to 400 degrees F.
Rinse and scrub the outsides of the beets. Carefully slice the beets into 1/16-inch-thick rounds using a mandolin slicer. Place in a large bowl and drizzle the olive oil over, turning to coat evenly. Lay out a single layer of beet slices on 2 baking sheets and sprinkle with freshly cracked pepper (don't add the salt at this point, as it will make the chips less crisp).
Bake for 30 to 45 minutes for a regular oven, rotating the sheets from top to bottom halfway through. Bake for 12 to 15 minutes for a convection oven, rotating halfway through. The beets will be darkened around the edges and be mostly crisp; they will firm up a little bit more once cooled. Let the chips cool, and then transfer to a bowl. Repeat with the remaining beet slices. When they are all cooked, toss with salt to taste.
For the guacamole: Mash the avocados with a fork or potato masher in a large bowl until slightly chunky. Stir in the cilantro, lime juice, tomatoes and onions. Season the guacamole with salt and pepper and garnish with olive oil.
Serve a mound of guacamole in a bowl. Serve with the beet chips.
Beet slices will shrink by almost half once baked. Use the largest beets that will fit on your mandolin slicer. Cilantro can be a polarizer - either you love it or you hate it. For less cilantro punch, feel free to leave it out, or just top at the end as garnish instead of mixing it in.